Control system



NOV. 5, 1940. H, a LA RQQUE 2,220,415

CONTROL SYSTEM Filed Jan. 28, 1935 v tCE/VE-l SCEIVE-2 SCENE-J SCENE-'4SCENE-J Inventor: Harold B. La Roque,

His Attorney. f

Patented Nov. 5, 1940 CONTROL SYSTEM.

Harold B. La Roque, Scotia, N. Y, assignor to General Electric Company,a corporation of New York Application January 2a, 1933, Serial No.654,075

18 Claims.

This invention relates to control systems, more particularly to systemsfor controlling the magnitude of the load of an electric circuit and ithas for an object the provision of a simple reliable and eflicientsystem of this character. More specifically, the invention relates tosystems in which means are provided for presetting a plurality of loadvalues for the load circuit, and a particular object of my invention isthe provision of means for gradually varying the load from one value toany of a plurality of succeeding preset values. A more specific objectof the invention is the provision of means for gradually varying theload from one value to another without any sliding or commutatingresistance contacts.

The invention has particular utility in its application to theatredimming systems in which pro- 'vision is made for presetting a pluralityof lighting scenes, and accordingly a specific object of 20 theinvention is the provision of means for gradually fading the presentscene into any one of a plurality of succeeding preset scenes.

In carrying the invention into effect in one form thereof a plurality ofcontrol circuits are provided for presetting a corresponding pluralityof load values for the load circuit and means are provided for varyingthe load from one value to another together with means for selectivelyconnecting the control circuit with the load varying means so that theload is varied from its present or instantaneous value to any presetvalues. A specific form of the invention embraces suitable electricvalve apparatus provided with a control grid for directly controllingthe magnitude of the load circuit, together with a plurality of presetcontrol circuits connected to the grid.

A variable saturable reactor provided with a magnetization controlwinding is connected to each control circuit, and a variable voltagedrop device, for example a variable reactance device,

is included in each control circuit for presetting a load value for the,controlled load circuit. Means for varying the load are provided in theform of means for varying the current flowing in the control windings ofthe saturable reactors, and suitable switching means are provided forselectively connecting the control windings to the load varying meansthus providing variation of the load from its present value to any ofthe plurality of preset values.

. In illustrating the invention in one form thereof it is shown asembodied in a theatre dimming system. For a better and more completeunderstanding of the invention reference should now be had to thefollowing specification and to the PATENT orrica accompanying drawing,the single figure of which is a simple schematic diagram of anembodiment of the invention applied for controlling a single circuit ofa theatre illumination system.

Referring now to the drawing, anindividual 5 lamp circuit illustrated ascomprising a plurality of lamps I0 is supplied from a suitable source ofalternating current represented in the drawing by the supply lines IIand H. \The illumination system of a modern theatre comprises a large 1number of individual circuits, for example, often more than one hundredand thus the circuit Il) may be taken as representing any of theplurality of individual circuits of the entire system.

For the purpose of varying the intensity of illu- I mination of the lampII) a variable saturable reactance device I3 is included in seriesrelationship between the lamp and the supply source. This saturablereactor comprises a reactive winding illustrated as two coils |3a andI31, connected I0 in series relationship and wound upon a suitable core(not shown) together with a direct current control winding l3 also woundupon a leg of the core. The reactive windings I33 and I3}, are connectedbetween the lower side of the lamp circuit and the lower side l2 ofthesupply source and the lamp circuit itself is readily traced from theupper side ll through the lamp, the reactive windings I38 and I31, tothe lower side of the supply source.

As is well understood the intensity of illumination of the lamp may bevaried byvarying the reactance of the reactive windings Na and l3b. Forthis purpose direct current of variable magnitude is supplied to thecontrol winding He 35 from a suitable source represented in the drawingby the electric valve l4. Although this valve may be of any suitabletype it is preferably of the three electrode type, into the envelope ofwhich a small quantity of an inert gas such for 4 example as mercuryvapor ,is introduced after exhaust. The presence of the gas within thetub'e serves to convert the usual pure electron discharge into an arcstream thus constituting the valve an electrostatically or gridcontrolled arc rectifier. The plate or anode of the valve I4 isconnected to one terminal of the control winding I30 whilst its cathodeis connected to the lower supply line l2. As thus connected, the valvel4 serves to rectify one-half of the wave of the alternating voltageimpressed upon the control winding 3c by the source H, H. For thepurpose of sustaining the current flow in the control winding during theidle or inverse half-cycle a second valve I5 is provided. This valve isillustrated as a two-electrode valve having its plate or anode connectedto the same terminal of the control winding |3c as that to which theanode of the valve I4 is connected and having its cathode connected tothe opposite terminal of the control winding by means of the conductorI6. The cathodes of both valves I4 and I5 are illustrated as being ofthe incandescent filamentary type heated to the required degree ofincandescence by ineansof electricenergy supplied from the source II, I2through the supply trans-l,

former H to the secondary winding I18 and I'll; of which the cathodes ofthe electric valves I4 and I5 are respectively connected as illustrated.

As is well understood, the average value of the current flowing in theoutput circuit of a gas filled electric valve such as the valve I4 maybe varied by varying the voltage applied to its controlgrid. Forexample, when the grid voltage is sufiiciently positive, the currentfiowingin the anode circuit is maximum whilst when the grid is biasedsufficiently negative the anode current is minimum or zero. Forintermediate grid bias voltage values the current flowing in the anodecircuit has corresponding intermediate values. For the purpose ofapplying a suitable D. C. voltage to the grid of the electric valve I4,the grid circuit of the latter is connected to the alternating voltagesupply source II, 12 through a suitable rectifying device I8 illustratedas a hot cathode type rectifier, and the grid of the'valve I4 isconnected to the lower supply side of the supply source I2 by means ofthe conductor 20 with suitable filtering devices 2I and 22 included incircuit for the purpose of smoothing the ripple of the rectified voltagewave. The left hand anode of the rectifier I8 is connected with a bus 23which in turn is connected through certain control circuits to the uppersupply line II whilst the mid point of the cathode supply-winding lie isconnected to a point between the filters 2i and 22 and thence throughthe filter 2I to the lower supply line I2. Thus, it will be clear thatthe rectifier I8 rectifies one-half of the voltage wave and that thisrectified voltage is applied to the grid circuit of the electric valve I4.

In order to provide for presetting a plurality of illuminationintensities for the lamp circuit III, 'a plurality of control circuits24, 25, 26, 21 and 28 are connected to the bus 23 which as previouslypointed out is connected through the rectifier I8 to the grid circuit ofthe electric valve I4. Although five of these control circuits areillustrated in the drawing, it will be clear that this number may beeither-more or less than five' depending upon the number of lightingscenes that it is desired to preset. The control circuit 24 comprises avariable voltage drop device illustrated as a variable reactance 30 anda capacitance 3| connected in parallel together with a capacitance 32connectedin series relationship. The upper terminal 'of the variablereactance winding 30 is connected to the upper supply line II when theswitch 33 is operated iof the reactance.

is connected through the rectifier I8 and the filter 2| to the lowersupply line I2. Thus it will be .seen that the reactive winding 35a isconnected in parallel relationship with the capacitance 32. A suitablefixed capacitance 31 is connectedbetween the right hand terminal of thecapacitance 32 and'the bus 23 and is thus connected in parallelrelationship with the reactive winding 35:; of the saturable reactor.The reactance of the variable reactance 30 may be varied as desired,preferably by varying the position of a core (not shown) within thesolenoid winding The value of this reactance, as well as the reactivevoltage drop across the winding 30 is preferably indicated by theposition on a calibrated scale of an indicating member (not shown)attached to the movable solenoid core.

The remainingcontrolxcircuits 25, 26, 21 .and 28 are in all respectsidentical with the, control circuit '24 and consequently a repetition ofthe above detailed description ofthe circuit 24 is omitted since thecircuit details of these remaining control circuits will be readilyunderstood .from the above-description of the control circuit '24.

When the saturable reactors 35, 40,, 42, and 43 are unsaturated, i. e.,-the reactanceand likewise the reactive drop of the reactive winding isvery high with the result that the greater portion, in factsubstantially all, of the voltage drop across the capacitance 32 appearsacross the reactive windings of the saturable reactors 35, 40, 4|, 42,and 43, so that the voltage applied to the grid of the valve I4 duringthe positive halfcycle of the voltage rectified by the rectifier I8 issubstantially the voltage of the lower supply line I2. During thepositive half-cycle of the voltage rectified by the rectifier I8 thevoltage of the line I2 is negative and consequently the voltage appliedto the grid of the valve I4 is also negative with the result that thevalve is nonconducting if the saturable reactors 35, 40, M, 42 and 43are unsaturated. However, if the saturation of any of these saturablereactors is increased the voltage drop across its reactive winding isdecreased with the result that the voltage applied-to the grid of thevalve I4 is made increasingly positive and the valve I4 becomesconducting. The magnitude of the current flowing in the anode circuit ofthe valve I4 is thus caused to vary in accordance with variations in thesaturation of the saturable reactances of the control circuits.

For the purpose of varying the saturation and the reactive voltage dropof the saturable reactors, suitable current varying means are providedfor varying the current supplied to the direct current magnetizationcontrol winding of these reactors and suitable selective switching means45 are provided for selectively connecting these magnetization controlwindings with the current varying means 44.

Although the current varying means 44 may be of any suitable type it isillustrated as electric valve apparatus comprising a pair of valves 46and a second pair of valves 41 under the control of a suitable controldevice illustrated as rotary induction apparatus 48. The valves 46 and41 may be of any suitable type but like valve I4 are preferably of thethree-electrode vapor filled type. 75

The valves 46 and 41 are supplied from anv suitable source ofalternating current, represented in the drawing by the conductors 50connected through the supply transformer As shown, the anodes of thepair of valves 46 and the pair of valves 41 are connected in parallel tothe opposite terminals of the secondary winding of the supplytransformer 5i. The mid-point of this secondary winding is connected bymeans of the conductor 52 to a bus 53 which in turn is connected to theupper terminal of each of the direct current control windings of thesaturable reactors 35, 40, 4|, 42 and 43 and the cathodes of one or theother of the pair of valves 46, 41 are connected through conductors 54or 55 to the' opposite or lower terminals of these direct currentmagnetization control windings through the selective switching apparatus45.

The control device 48 is illustrated as a voltage regulating devicehaving a stator winding 82. and two rotor windings 48b and 48c wound atright angles upon the perpendicular legs of the rotor core punchings.The stator winding 48a is supplied with two-phase alternating voltagefrom a suitable source represented in the drawing by the four supplylines 56 and 55a. The control grids of the pair of electric valves 46are. connected to the opposite terminals of the secondary winding of thegrid transformer 51, the opposite terminals of the primary winding ofwhich are connected to the rotor winding 48c of the control device.Similarly, the grids of the pair of electric valves 41 are connected tothe opposite terminals of the secondary winding of the grid transformer58, the terminals of the primary winding of which are connected to therotor winding 4%. When alternating current flows in the winding 48a ofthe control device 48, voltages are induced in the rotor windings 48band 68c and sincev these rotor windings 48b and 48a are arranged atright angles with each other the induced voltages will have quarterphase relationship with each other, that is to say that when the voltageinduced in one of the windings is maximum, that induced in the otherwinding is minimum and vice versa. When one of these rotor windings hasits axis parallel with the axis of the rotating magnetic field due tothe stator winding, the induced voltage in this winding is maximum andsince the axis of the other winding is atright angles with the magneticfield of the stator winding, its induced voltage at this instant isminimum or zero. Thus, it will be seen that if the axis of one of therotor windings of the control device 48 is parallel with the magneticfield of the stator winding during the positive half cycle of anodevoltage, its induced voltage will be maximum and the pair of electricvalves to the control grids of which it is connected, will conductmaximum current and that the other rotor winding will have zero inducedvoltage and consequently the electric valves to the control grids ofwhich it is connected will be non-conducting and inactive. By rotatingthe rotor of the control device one-quarter turn, the voltage in onerotor winding will be gradually shifted out of phase with the valveanode voltage whilst the voltage in the other winding will be graduallyshifted in phase with the valve anode voltage and similarly the currentsupplied by the active or conducting pair of valves is gradually reducedfrom maximum to zero, whilst the current supplied by the other pair ofvalves is gradually increased from zero to maximum value.

' winding With the above understanding of the apparatus and itsorganization in the complete system, the operation of the system itselfwill readily be understood from the detailed description which follows:

The switches 33, 51, 58, 60 and 6! are operated to their right handpositions to connect the control circuits 24-28 inclusive across thesupply lines ll, l2. A plurality of lightingscenes, i. e., a pluralityof difierent illumination intensities of the lamp circuit in are thenpreset for the desired illumination intensity by presetting theeffective values of the presettable reactances 36, a, 30b, 36c, and 30ato the desired values as observed upon their cooperating dials. At thistime the movable contacts of the selective switching mechanism 45 areall in the open position in which they are illustrated and consequentlythe reactive drops across the saturable reactors in the control circuits24-28 inclusive are so large that the grid voltage of the electric valveI4 is negative and'the valve is non-conducting. Since no current flowsin the control winding of the saturable reactor it, the reactive dropacross its reactive winding i3a, i3b is maximum and the lamps ill areextinguished. The induced voltage in winding 48 of control device 48 isout of phase with the valve anode voltage and the pair of valves 46 isbiased to cut oil and therefore inactive.

To present the first of the preset scenes, 1. e.,

scene I, movable contact 639. of the selective increasing the directcurrent flowing in the control winding b. This increase in the currentflowing in the control winding 35b decreases the reactance across thereactive winding 3% and consequently increases the voltage applied tothe grids of the electric valve M which in turn results in increasingthe current flowing in the control creasing the reactive voltage dropacross'the reactive winding He and 53b and increasing the intensity ofillumination of the lamp 13. When the rotor of the control device hasbeen rotated one-quarter turn, the grid and anode voltages the winding48b is rotated into a position at right angles so that the pair ofvalves 41 is rendered inactive.

Assuming now that it is desired to present scene 2 and gradually tomerge or fade scene I into scene 2, the movable contact 64b of theselective switching mechanism is depressed to complete the connectionsof the control winding b of saturable reactor 46 'to the pair ofelectric valves 41. The scene fader 48 is then rotated one-fourth turnand the conducting valves 46 are gradually rendered non-conductingwhilst I30 of the saturable reactor l3, de-- the inactive valves 41 aregradually rendered conducting. As the current is decreased in thecontrol winding 35b, the grid voltage of the electric valve I4 iscorrespondingly decreased but simultaneously the increasing current inthe control winding 40!; of saturable reactor 40 increases the gridvoltage of the valve 14 so-that when the into scene -I.

by the preset reactance device of scene 2 con-' trol circuit 25. It willbe clear that since the reactive voltage drop across the reactivewinding 35:: is increased at the same time that the reactive voltagedrop across the reactive winding 40a is decreased the grid voltage ofthe electric valve i4 is gradually changed from the value preset by thereactance 30 to the value preset by the reactance 30a. As a result theillumination intensity of the lamp circuit In is gradually changed fromthe-value preset for the first scene to the value preset for the secondscene.

In a similar manner, it it is'now desired to present scene 3, themovable contact 53c is depressed to complete connections of the controlwinding 4Ib of the saturable reactor M to the scene I, is released. Thescene fader 48 is now rotated one-quarter turn and the illuminationintensity of the lamp circuit I is gradually merged or faded from thevalue preset for scene i 2 to the value preset for scene 3.

It is' not necessary that the scenes be presented in progressivenumerical order, i. e., it is not necessary that the scenes be presentedin the orders I, 2, 3, 4, 5, etc. For example, let it be assumed thatscene 3 is being presented and is thus the present scene and it isdesired instead of presenting scene 4 as the next scene that scene I bereestablished. 'In order to repeat scene i 'it will be clear that thecontact 63a. cannot be depressed since this would'connect the controlwinding 35!) to the pair of valves 46 which at this point are now activein maintaining scene 3. Therefore, it will be clear that the switchcontact 64a must be depressed, releasing the switch contact 64b,previously depressed for-scene 2, and connecting the control winding 35bto' the pair of valves 41. By rotating the scene fader 48 onefourthturnas before, scene 3 is gradually faded Thus it will be clearthat anyscene may be directly faded in straight line variation into any one of aplurality of preset scenes, without passing through the illuminationintensity values for any other scenes in the set up.

For the purpose of simplification, but a single lamp circuit is shown inthe drawing. It will be clear however that any number of circuits may becontrolled in exactly the same manner as that Although in accordancewith the provisions of the patent statutes, this invention is describedas embodied in concrete form, the invention is not limited to thespecific apparatus and connections described and illustrated sincealterations and modifications will readily suggest themselves'to personsskilled in the art without departing from the true spirit of thisinvention or the scope of the annexed claims.

; What I claim as new and desire to secure by Letters Patent of theUnited States, is:

i. In a control system for a load circuit, means for controlling themagnitude of said load, means for presetting a plurality oi load valuescomprising a plurality of control. circuits each including a variablevoltage device for applying preset voltages to said load controllingmeans, and. means for varying said load from any preset value to anothercomprising means for simultaneously varying one of said voltages'from apreset value to a minimum value and another of said voltages from aminimum-value to a'preset value, and means for selectively connectingany two of said circuits to said voltage varying means.

2. A control system comprising a translating circuit, a load deviceincluded in said circuit, means for controlling the voltage applied tosaid load device, means for presetting a plurality of voltage values forsaid load device comprising a plurality of variable voltage devices forapplying a plurality of preset control voltages to said load voltagecontrol means, means for varying said load voltage from any preset valueto another comprising means for varying one of said control voltagesfrom a preset value to a minimum value and another of said controlvoltages from a minimum value to a preset value, and'means forselectively connecting any two of said variable voltage devices to saidcontrol voltage varying means. I

3. An illumination control system comprisin a lamp circuit, means forcontrolling the intensity of illumination of said circuit comprisingelectric valve apparatus provided with a control grid, control means forsaid valve apparatus comprising a plurality of control circuitsconnected to said grid means for presettlng a plurality of sceneillumination intensities comprising a separate variable voltage deviceconnected to each of said control circuits for applying preset voltagesto said grid, means for fading the intensity of illumination of saidlamp circuit. from one preset value to another comprising means foroppositely varying said grid voltage between two of said preset valuesand a minimum voltage and switching means for selectively connecting atleast two of said control circuits to said voltage varying means.

4. In a system for controlling a load circuit, means for controllingsaid circuit comprising electric valve apparatus provided with a controlgrid, control means for said valve apparatus comprising a plurality ofsaturable core reactance devices each connected to said grid and eachprovided with a control winding, and means for selectively energizingsaid windings.

5. In a system for controlling a load'circuit, control means for saidcircuit'comprising a plurality of variable reactance devices eachprovided with a control winding, means for selectively controlling theenergization of said windings, and means for simultaneously andoppositely ,varying the energization of at least two of said windingscomprising a pair of electric valves each having an output circuit,means for connecting any two of said control windings to said outputcircuit and control means for said valves for simultaneously andoppositely varying the current in said output circuit.

6. In a. system for controlling a load circuit, means for controllingsaid circuit comprising electric valve apparatus provided with a controlgrid, control means for said valve apparatus comprising a plurality ofreactance devices connected to said grid, and means for simultaneouslyand oppositely varying the reactance of said devices.

'7. In a system for controlling a load circuit, means for controllingthe magnitude of the load of said circuit comprising a plurality ofreactive circuits, means for presetting the effective reactance valuesof said circuits individually and means for oppositely varying thereactance of at least two of said circuits between said efiective valuesand a minimum value comprising a pair of electric valves each providedwith a control grid and an output circuit, switching means forrespectively connecting any two of said reactive circuits to said outputcircuit and a rotary induction device connected to said grid circuit forsimultaneously and oppositely varying the current in said outputcircuit.

8. In a system for controlling a load circuit, control means for saidcircuit comprising a plurality of reactive circuits each including avariable reactance device provided with a magnetization control winding,means for individually presetting the reactance values of said reactivecircuits, and means for oppositely varying the energization of at leasttwo of said windings comprising a pair of electric valves each providedwith a control grid and an output circuit, switching means forrespectively connecting any two of said control windings with saidoutput circuit and a phase shifting device connected to said inputcircuit for simultaneously and oppositely varying the current in saidoutput circuit.

9. In an illumination control system, the combination with a lightingcircuit of a plurality of presettable devices for presetting a pluralityof illumination intensities for said lighting circuit, a fading devicefor simultaneously renderingone of said presettabledevices inactive andanother of said presettable devices active thereby to fade the intensityof illumination of said lighting circuit from one of said preset valuesto another, and a selector switching device for selectively connectingany two of said presettable devices to said fading device thereby toprovide for fading the intensity of illumination of said lightingcircuit from any one of said preset values to any other of said presetvalues.

10. An illumination control system comprising in combination, a lightingcircuit, a variable dimming means for varying the intensity ofillumination of said lighting circuit, a plurality of control circuitsfor presetting a plurality of illumination intensities for said lightingcircuit, each of said control circuits including a device having awinding connected to said dimming means and a cooperating controlwinding, a fading device for simultaneously rendering a first of saidcontrol circuits inactive and a second of said control circuits activethereby to fade the intensity of illumination of said lightingcircuitfrom the intensity preset on said first circuit to the intensitypreset on said second circuit, and a selector switching device forselectively completing connections between any two of said controlwindings and said fading device thereby to provide for fading theintensity of illumination of said lighting circuit from any one of saidpreset values to any other of said preset values. j

11. In an illumination control system, a lamp circuit, means forcontrolling the'intensity of illumination of said circuit comprisingelectric valve apparatus provided with a control grid, control means forsaid apparatus comprising a plurality ofreactive circuits connected tosaid grid and each including a variable reactance device, means forpresetting a plurality of scene illumination intensities for said lampcircuit comprising means for presetting effective reactance values forsaid reactive circuits, means for selectively energizing said reactivecircuits and means for fading the illumination intensity of said lampcircuit for one scene to the intensity for a succeeding scene comprisingmeans for oppositely varying. the reactance of said reactive circuitsbetween said effective values and minimum values.

12. An illumination control system comprising in combination with a lampcircuit, means for controlling the intensity of illumination of saidcircuit comprising electric valve apparatus provided with a controlgrid, control means for said valve apparatus comprising a plurality ofreactive circuits each including means for presetting the effectivereactance value of said circuit and a variable reactance device having areactive winding connected to said grid and a control winding, and meansfor fading the illumination intensity of said lamp circuit for one sceneto the intensity for a succeeding scene comprising means for varying theenergization of said control windings.

13. In an illumination control system for a lamp circuit and the like,means for controlling the intensity of illumination of said circuitcomprising electric valve apparatus provided with a control grid,control means for said valve apparatus comprising a plurality ofreactive circuits, a separate variable reactance device included in eachof said circuits each having a reactive winding connected to said gridand each having a control winding, means for presetting a plurality oflighting scenes comprising means for presetting effective reactancevalues for each of said reactive circuits, means for selectivelyactivating at least any. two of said reactive circuits, and scene fadingmeans comprising a device connected to said control windings forsimultaneously increasing the reactance of one of. said activatedcircuits and decreasing the reactance of another of said circuits.

14. In an illumination control system for a lamp circuit and the like,means for controlling the intensity of illumination of said circuitcomprising electrlc valve apparatus provided with a control grid,control means for said valve apparatus comprising a plurality of controlcircuits connected to said grid, a separate reactance device for. eachof said control circuits, each of said devices having a reactive w ndingconnected to its associated circuit and a direct current controlwinding, means connected to each of said control circuits for presettinga plurality of scene illumination intensities for said lamp circuit, anda scene fader comprising means for varying the direct current of saidcontrol windings.

15. In an illumination control system, a lamp circuit, illuminationintensity control means for said circuit comprising electric valveapparatus provided with a control grid, control means for said valveapparatus comprising a plurality of control circuits connected to saidgrid, 9. separate variable reactance'device for each of said controlcircuits, each device having a reactive winding connecting itsassociated control circuit and a direct current magnetization controlwinding, and scene fading means comprising electric valve apparatusconnected to vary the current flowing in saidwindings.

16, An illumination'control system comprising in combination with a lampcircuit, illumination intensity control means for said circuitcomprising electric valve apparatus provided with a control grid, meansfor presetting a plurality of scene illumination intensities for saidlamp circuit comprising a'plurality of control circuits connected tosaid grid, a separate variable reactance device for each of said controlcircuits, each of said devices comprising a reactive winding connectedto its associated control circuit and a direct current control winding,scene fading means comprising electric valve apparatus provided with acontrol grid and means connected thereto for varying the current in saiddirect current windings, and

switching means for selectively connecting any two of said directcurrent windings to said scene fading valve apparatus.

1'7. An illumination control system comprising in combination a lampcircuit, control means for said circuit comprising electric valveapparatus provided with a control grid, control means for said valveapparatus comprising a plurality of control circuits connected to saidgrid, means for presetting a plurality of scene illumination intensitiesfor said lamp circuit comprising a plurality of grid voltage controldevices one connected to each of said control circuits, a plurality ofvariable reactance devices each having a reactive winding connected toone of said control circuits and a direct current control winding, ascene fader comprising electric valve apparatus arranged for connectionto said control windings and provided with a control grid and a controldevice connected thereto for causing said scene fading valve apparatusto increase the current comprising a plurality of control circuitsconnected to said grid, means for presetting a plurality ofsceneillumination intensities comprising a separate presettablereactance device in each of said control circuits for presetting aplurality of voltages for application to said grid, a separate variablereactance device for. each of said control circuits, each of saiddevices having a reactive winding included in its associated controlcircuit and a direct current control winding, scene fading meanscomprising means for varying the current in said control windings tovary said grid voltage from one preset value to another, and switchingmeans for selectively connecting said control circuits to said scenefading means. 4

HAROLD B. LA ROQUE.

